Sintered R-T-B based magnets (where R is at least one rare-earth element which always includes at least one of Nd and Pr; T is Fe, or Fe and Co; and B is boron) are known as permanent magnets with the highest performance, and are used in voice coil motors (VCM) of hard disk drives, various types of motors such as motors for electric vehicles (EV, HV, PHV, etc.) and motors for industrial equipment, home appliance products, and the like.
A sintered R-T-B based magnet is composed of a main phase which mainly consists of an R2T14B compound and a grain boundary phase that is at the grain boundaries of the main phase. The R2T14B compound, which is the main phase, is a ferromagnetic material having a high saturation magnetization and anisotropy field, and provides a basis for the properties of a sintered R-T-B based magnet.
There exists a problem in that coercivity HcJ (which hereinafter may be simply referred to as “coercivity” or as “HcJ”) of sintered R-T-B based magnets decreases at high temperatures, thus causing an irreversible thermal demagnetization. For this reason, sintered R-T-B based magnets for use in motors for electric vehicles, in particular, are required to have high HcJ at high temperatures, i.e., to have higher HcJ at room temperature.